JP7779401B2 - Slab determination method, slab manufacturing method, and steel plate manufacturing method - Google Patents

Description

The present invention relates to a method for assessing the risk of slabs splitting into two pieces, a method for manufacturing slabs using the assessment method, and a method for manufacturing steel plates.

In recent years, in the automotive industry, large amounts of elements such as Si, Mn, and Ti have been added to steel in order to reduce the weight of car bodies and improve workability. Steel containing large amounts of elements such as Si, Mn, and Ti frequently suffers from the problem of splitting into two pieces during rolling, significantly reducing productivity. Here, splitting into two pieces refers to a phenomenon in which a crack appears in the center of the steel plate's thickness, causing the hot-rolled steel plate to split into two pieces.

Even if splitting does not become apparent at the hot-rolled steel sheet stage, if the hot-rolled steel sheet is shipped with an internal crack, splitting may occur during pressing or bending. For these reasons, it is necessary to eliminate defects that cause splitting.

The cause of double cracks is thought to be the oxidation of cross-sectional defects in the slab (cross-sectional cracks present on the cut surface or voids in the final solidification area) in the heating furnace. However, not all slabs with cross-sectional defects will develop double cracks, and the incidence of double cracks varies greatly depending on the type of steel.

As a technique for preventing splitting, Patent Document 1 discloses a continuous steel casting method in which the slab is forcibly pressed down at the end of solidification to close voids in the final solidification area. Patent Document 2 discloses a method for reducing the secondary cooling flow rate to prevent the containment of residual liquid phase even in unsteady areas.

Japanese Patent Application Laid-Open No. 2003-94154 Japanese Patent Application Laid-Open No. 2003-334651

Conventional technology cannot explain the difference in the occurrence of two-ply cracks depending on the steel type, and has the following problems. Using the method disclosed in Patent Document 1 makes it possible to eliminate cross-sectional defects in slabs. However, the reaction force generated when forcibly rolling down the slab increases, significantly reducing the lifespan of the rolls and segments. Therefore, applying this method to all steel types creates an economic issue in that slab manufacturing costs increase.

In order to affect the solidified shell thickness using the method disclosed in Patent Document 2, it is necessary to reduce the amount of cooling water from an early stage of solidification, which increases inter-roll bulging due to weakened cooling, and there is a risk of generating other defects such as internal cracks. For this reason, if this method were applied to all steel types, there is a risk of increasing unnecessary risks.

The present invention was made in consideration of these circumstances, and its purpose is to provide a slab assessment method that can determine the risk of a slab produced using molten steel splitting into two pieces based on the component concentrations of the molten steel produced in a continuous casting device.

The means for solving the above problems are as follows.
[1] A method for determining a slab containing Ti, C, Si, and Mn as components and produced using a continuous casting machine, the method comprising: a first step of determining an index of the liquid phase ratio of scale formed in the slab using the component concentrations of molten steel poured into a mold of the continuous casting machine; and a second step of determining that the slab is at risk of splitting into two pieces if the index of the liquid phase ratio of the scale is equal to or greater than a predetermined threshold.
[2] The method for evaluating a slab according to [1], wherein the index of the liquid phase ratio of the scale is an index determined by the Ti concentration, the C concentration, the Si concentration, and the Mn concentration of the molten steel.
[3] The method for determining a slab according to [1] or [2], wherein the index of the liquid phase ratio of the scale is an index determined by the following formula (1):
f=A _Ti1 × [Ti] × {A _C × [C] + A _Si × [Si] + A _Mn × [Mn] + A _Ti2 × [Ti] + A _Si/Mn × ([Si]/[Mn]) ^0.5 }... (1)
In the above formula (1), f is an index (-) of the liquid phase ratio of the scale, [Ti] is the Ti concentration (mass%) of the molten steel, [C] is the C concentration (mass%) of the molten steel, [Si] is the Si concentration (mass%) of the molten steel, [Mn] is the Mn concentration of the molten steel, and A _Ti1 , A _C , A _Si , A _Mn , A _Ti2 and A _Si/Mn are parameters.
[4] In the second step, if the index of the liquid phase ratio of the scale is less than a predetermined threshold, the slab is judged to be a slab with no risk of splitting into two pieces. A slab judgment method according to any one of [1] to [3].
[5] A method for producing a slab using a continuous casting device, in which, when a slab is determined to be at risk of splitting into two pieces by the slab determination method described in any one of [1] to [3], the roll opening of the reduction rolls of the continuous casting device is changed so that the solid phase ratio at the center of the width and thickness of the slab in the casting direction is in the range of more than 0.0 and not more than 1.0, to lightly reduce the slab.
[6] A method for manufacturing a slab using a continuous casting device, wherein, when a slab is determined to be at risk of splitting into two pieces in the slab determination method described in any one of [1] to [3], the temperature difference in the width direction of the slab is measured in the range from the position where the slab support rolls are no longer installed to the position where the slab is cut to determine whether or not there is a cross-sectional defect.
[7] A method for manufacturing a slab using a continuous casting device, wherein, in the slab judgment method described in any one of [1] to [3], if the slab is judged to be at risk of splitting into two pieces, the cut surface of the slab is imaged to generate image data, and the presence or absence of cross-sectional defects is judged using the image data.
[8] The method for manufacturing a slab according to [7], wherein the cut surface is imaged using an imaging device having an IR cut filter.
[9] A method for manufacturing a slab according to any one of [6] to [8], in which a gap in a cross-sectional defect of a slab determined to have a cross-sectional defect is welded.
[10] A method for manufacturing a steel plate in which a slab is heated and hot-rolled to manufacture a steel plate, wherein a slab determined to have no risk of splitting into two pieces by the slab determination method described in [4] is heated and hot-rolled. A method for manufacturing a steel plate.
[11] A method for producing a steel plate by heating a slab and hot rolling it,
A method for producing a steel plate, comprising heating a slab produced by the method for producing a slab according to [5] and hot rolling the slab.
[12] A method for manufacturing a steel plate in which a slab is heated and hot-rolled to manufacture a steel plate, the method comprising heating and hot-rolling a slab determined to be free of the cross-sectional defect by the slab manufacturing method according to any one of [6] to [8].
[13] A method for producing a steel plate by heating a slab and hot rolling it to produce a steel plate, wherein the slab produced by the slab production method according to [9] is heated and hot rolled.

According to the present invention, by using an index of the liquid phase ratio of scale obtained from the component concentrations of molten steel, it is possible to determine the risk of slabs cracking into two pieces when produced using that molten steel. By determining the risk of cracking into two pieces in this way, it becomes possible to select and address slabs determined to be at risk of cracking into two pieces, thereby minimizing increases in slab production costs and the risk of unnecessary risk.

FIG. 1 is a schematic side view of a continuous casting apparatus capable of carrying out the slab evaluation method and slab manufacturing method according to this embodiment. FIG. 2 is a graph showing the relationship between the index f of the liquid phase ratio of scale and the incidence of two-ply cracks. FIG. 3 is a graph showing the relationship between the temperature difference in the width direction and the cross-sectional defects of the slab.

The present invention will now be described in detail through embodiments of the present invention. The following embodiments are intended to be preferred examples of the present invention, and the present invention is not limited to these embodiments in any way.

Figure 1 is a schematic side view of a continuous casting apparatus 100 in which the slab evaluation method and slab manufacturing method according to this embodiment can be implemented. The continuous casting apparatus 100 has a mold 10, a tundish 12 installed above the mold 10, and multiple pairs of cast piece support rolls 32 arranged in a row below the mold 10. A sliding nozzle 14 for adjusting the flow rate of molten steel 18 is installed at the bottom of the tundish 12, and an immersion nozzle 16 is installed on the underside of this sliding nozzle 14.

Molten steel 18 is poured into the mold 10 through the submerged nozzle 16. The molten steel 18 poured into the mold 10 solidifies as heat is removed from the inner surface of the mold 10, forming a solidified shell 20. This results in the formation of a cast 24 having the solidified shell 20 as its outer shell and an unsolidified layer 22 made of molten steel 18 inside.

Below the mold 10, several pairs of strand support rolls 32, including support rolls 26, guide rolls 28, and pinch rolls 30, are provided. Of these, the pinch rolls 30 support the strand 24 and also serve as drive rolls for withdrawing the strand 24. Spray nozzles (not shown), such as water spray nozzles or air mist spray nozzles, are provided in the gaps between adjacent strand support rolls in the casting direction, thereby forming a secondary cooling zone. The strand 24 is cooled as it is withdrawn by the cooling water sprayed from the spray nozzles in the secondary cooling zone (hereinafter also referred to as "secondary cooling water"), reducing the internal unsolidified layer 22 and growing the solidified shell 20. The strand 24 is then appropriately cooled, solidifying the unsolidified layer 22 until the strand 24 is completely solidified. A plurality of transport rolls 34 for transporting the cast strand 24 are provided downstream of the strand support rolls 32, and a strand cutter 36 for cutting the cast strand 24 into slabs 38 of a predetermined length is provided above the transport rolls 34.

Upstream of the solidification completion position 40 of the slab 24 in the casting direction, a soft reduction zone 42 is provided, allowing the roll spacing between the opposing guide rolls 28 to be adjusted. The guide rolls that make up the soft reduction zone 42 are configured so that the roll spacing can be adjusted, and by adjusting the roll spacing, the slab 24 can be soft reduced, or no soft reduction can be applied. In this embodiment, the soft reduction zone 42 is installed so that the slab 24 falls within the installation range of the soft reduction zone 42 until the solid fraction at the thickness center and width center of the slab 24 reaches at least 0.1 to 1.0. The solid fraction at the thickness center of the slab 24 refers to the solid fraction at the thickness center of the slab 24 excluding the width-wise ends of the slab, but may be represented by the solid fraction at the width-wise center and thickness center of the slab. Here, the solid fraction is an index showing the progress of solidification, and is expressed in the range of 0.0 to 1.0, with a solid fraction of 0.0 (zero) meaning that the material is not yet solidified, and a solid fraction of 1.0 meaning that the material is completely solidified.

The reduction gradient in the soft reduction zone 42 is expressed as the roll opening reduction amount (mm/m) per meter in the casting direction, and the reduction rate (mm/min) of the slab 24 in the soft reduction zone 42 is calculated by multiplying this reduction gradient (mm/m) by the casting speed (m/min). The soft reduction zone 42 reduces the slab 24 at a reduction rate of 0.2 mm/min to 1.0 mm/min. Spray nozzles for cooling the slab 24 are also arranged between each of the slab support rolls that make up the soft reduction zone 42. While Figure 1 shows an example in which only guide rolls 28 are arranged in the soft reduction zone 42, pinch rolls 30 may also be arranged in the soft reduction zone 42. The slab support rolls 32 arranged in the soft reduction zone 42 are also called "reduction rolls."

A thermal camera 44 is installed in the casting direction, ranging from the position where the strand support rolls 32 no longer exist to the position where the strand cutter 36 is installed, to measure the surface temperature of the strand 24 across its width. The thermal camera 44 measures the temperature difference across the width of the strand 24. By installing the thermal camera 44 in the range from the position where the strand support rolls 32 no longer exist to the position where the strand cutter 36 is installed, the surface temperature of the strand 24 can be measured without being affected by water vapor or steam. This makes it easy to measure the temperature difference across the width of the strand 24. In addition, an illumination device 46 is installed to illuminate the cut surface of the slab 38 cut by the strand cutter 36, and an imaging device 48 is installed to capture an image of the cut surface and generate image data.

In such a continuous casting apparatus 100, if the components of the slab 38 produced without soft reduction in the soft reduction zone 42 include Ti, C, Si, and Mn, the hot-rolled steel sheet produced from the slab 38 may split into two pieces. Splitting into two pieces is a phenomenon in which a crack appears in the center of the thickness during rolling or at the customer's site, causing the steel sheet to split into two pieces.

The inventors conducted extensive research into the causes of lamination in hot-rolled steel sheets produced by hot-rolling slabs 38 containing Ti, C, Si, and Mn. As a result, they discovered that lamination is closely related to the presence of cross-sectional defects in slabs 38 and a high liquid phase fraction in the scale formed in the high-temperature environment of a heating furnace. Therefore, even if cross-sectional defects exist in slabs 38, lamination will not occur in the hot-rolled steel sheets if the liquid phase fraction of the scale formed in the high-temperature environment of a heating furnace is low.

An index of the liquid phase ratio of scale formed in the high-temperature environment of a heating furnace can be determined using the Ti, C, Si, and Mn concentrations of the components of the molten steel 18. Therefore, in the slab determination method according to this embodiment, a first step is performed in which an index of the liquid phase ratio of scale is determined using the Ti, C, Si, and Mn concentrations of the components of the molten steel 18 poured into the mold 10. If the index of the liquid phase ratio of scale determined in the first step is equal to or greater than a predetermined threshold, a second step is performed in which the slab 38 is determined to be at risk of splitting into two pieces. Specifically, in the first step, the index of the liquid phase ratio of scale f is determined using the Ti, C, Si, and Mn concentrations of the molten steel 18 and the following equation (1):

f=A _Ti1 × [Ti] × {A _C × [C] + A _Si × [Si] + A _Mn × [Mn] + A _Ti2 × [Ti] + A _Si/Mn × ([Si]/[Mn]) ^0.5 }... (1)
In the above formula (1), f is an index (-) of the liquid phase ratio of scale. [Ti] is the Ti concentration (mass %) of the molten steel 18. [C] is the C concentration (mass %) of the molten steel 18. [Si] is the Si concentration (mass %) of the molten steel 18. [Mn] is the Mn concentration (mass %) of the molten steel 18. A _Ti1 , A _C , A _Si , A _Mn , A _Ti2 , and A _Si/Mn are parameters. (-) indicates dimensionless. For example, when the temperature inside the heating furnace is 1200°C, the parameters of the above formula (1) are A _Ti1 8.2, A _C is -0.7, A _Si is 2.0, A _Mn is -0.5, A _Ti2 is 23, and A _Si/Mn is 2.7.

In the second step, a slab whose scale liquid phase ratio index f calculated using the above formula (1) is equal to or greater than a predetermined threshold value is determined to be at risk of splitting into two layers. On the other hand, a slab whose scale liquid phase ratio index f is less than a predetermined threshold value may be determined to be at no risk of splitting into two layers.

Figure 2 is a graph showing the relationship between the index f of the liquid phase ratio of scale and the incidence of double cracks. The horizontal axis of Figure 2 represents the index f(-) of the liquid phase ratio of scale, which is a value calculated using the above formula (1) and the Ti concentration, C concentration, Si concentration, and Mn concentration of molten steel 18. The parameters used in formula (1) were A _Ti1 : 8.2, A _C : -0.7, A _Si : 2.0, A _Mn : -0.5, A _Ti2 : 23, and A _Si/Mn : 2.7.

The vertical axis of Figure 2 represents the incidence rate (%) of double-cracks. The incidence rate of double-cracks is the incidence rate (%) of double-cracks occurring in hot-rolled steel sheets produced by hot-rolling slabs cast without soft reduction using molten steel 18 with the component concentrations for which the scale liquid phase fraction index f was calculated. As shown in Figure 2, for slabs with a scale liquid phase fraction index f of 1.00 or more, the incidence rate of double-cracks in the hot-rolled steel sheets produced by hot-rolling the slabs was 0.2% or more. On the other hand, for slabs with a scale liquid phase fraction index f of less than 1.00, the incidence rate of double-cracks in the hot-rolled steel sheets produced by hot-rolling the slabs was 0%. From these results, the scale liquid phase fraction index f is calculated using the component concentrations of molten steel 18 (Ti concentration, C concentration, Si concentration, Mn concentration) and the above formula (1), and it is determined whether the double-crack risk index f is 1.00 or more. This makes it possible to determine the risk of the slab produced from the molten steel splitting into two pieces.

If the risk of double cracking can be determined in this way, slabs determined to have a risk of double cracking can be selected and addressed, thereby preventing increases in slab 38 manufacturing costs and unnecessary increases in risk due to dealing with slabs 38 that do not have a risk of double cracking. 1.00 used to determine the risk of double cracking is an example of a threshold value for the scale liquid phase ratio index f. The threshold value for the scale liquid phase ratio index f can be determined in advance by investigating the relationship between the scale liquid phase ratio index f and the occurrence rate of double cracking, as shown in Figure 2.

If a slab is determined to have a risk of splitting into two pieces using the slab determination method of this embodiment, and if a cross-sectional defect is present in the slab, there is a high possibility that a split will occur in the hot-rolled steel sheet produced by hot rolling. Therefore, in the slab manufacturing method of this embodiment, for slabs determined to have a risk of splitting into two pieces using the slab determination method, the continuous casting conditions are changed to prevent cross-sectional defects from occurring in the slab.

For example, it is preferable to reduce cross-sectional defects in the slab produced by narrowing the roll spacing of the reduction rolls in the soft reduction zone 42 of the continuous casting apparatus 100 to less than the thickness of the slab 24 and applying a soft reduction rate of 0.2 mm/min to 1.0 mm/min. This soft reduction is preferably applied so that the solid fraction at the center of the width and thickness of the slab 24 in the casting direction is greater than 0.0 and less than 1.0. The soft reduction may also be applied so that the solid fraction is greater than 0.0 and less than 0.7 (flow limit solid fraction). This prevents cross-sectional defects from occurring in the slab 38.

In this way, by changing the continuous casting conditions to produce a slab 38 without cross-sectional defects, even if scale with a high liquid phase ratio occurs in the high-temperature environment inside the heating furnace, it is possible to prevent the hot-rolled steel sheet produced by hot rolling the slab from splitting into two pieces.

On the other hand, if the slab is determined to have no risk of splitting into two pieces using the slab determination method of this embodiment, even if the slab has a cross-sectional defect, the splitting will not occur in the hot-rolled steel sheet produced by hot rolling the slab. Therefore, for slabs determined to have no risk of splitting into two pieces using the above slab determination method, it is preferable to change the continuous casting conditions so that the roll spacing of the guide rolls 28 in the light reduction zone 42 is widened to the thickness of the slab 24 so as not to apply a soft reduction to the slab 24. This suppresses the reduction in the lifespan of the guide rolls and roll segments in the light reduction zone 42, thereby suppressing an increase in the manufacturing cost of the slab 38.

As described above, even when a hot-rolled steel sheet is manufactured using a slab produced by the slab manufacturing method according to this embodiment, the steel sheet will not split into two pieces. Therefore, by heating the slab in a heating furnace and hot-rolling it in a rolling mill, hot-rolled steel sheet that does not split into two pieces during rolling or at the customer's site can be manufactured.

Next, we will explain the above formula (1) used to calculate the index f of the liquid phase fraction of scale. Slabs with a high liquid phase fraction of scale in the high-temperature environment of a heating furnace have a high frequency of occurrence of splitting in the hot-rolled steel sheet produced by hot rolling the slab. Liquid phase scale has a faster oxygen diffusion rate than solid phase scale, so it diffuses sufficiently within cross-sectional defects and oxidizes. When this liquid phase scale is elongated during hot rolling, splitting occurs during rolling of the hot-rolled steel sheet or at the customer's site. Furthermore, the inventors' research has confirmed that the liquid phase fraction of scale is particularly high when fayalite (Fe ₂ SiO ₄ ) is formed, and that the liquid phase fraction of scale increases further with the addition of Ti.

Equation (1) for calculating the liquid phase fraction index f of scale is a regression equation created by taking into account the degree to which the component concentrations of molten steel 18 affect the liquid phase fraction of the scale. The regression equation and its parameters were determined by first using thermodynamic calculations to calculate the liquid phase fraction of scale in a predetermined high-temperature environment within a heating furnace, and then taking into account the component dependency of this liquid phase fraction. For FeO-SiO ₂ -MnO-TiO ₂ scale, the melting point decreases as the composition approaches fayalite (Fe ₂ SiO ₄ ). Therefore, the higher the Si concentration in molten steel 18, the more likely low-melting-point scale is to form. On the other hand, the higher the amount of oxidizable additive elements C and Mn, the more difficult it is to form SiO ₂ . Therefore, the C concentration and Mn concentration parameters in the regression equation were set to negative values. Since Mn is added in large amounts, depending on the ratio with Si, the scale formed may switch to being mainly MnO. To take this into account, a term, Si concentration/Mn concentration, was introduced into the regression equation. Furthermore, Ti has the effect of lowering the melting point of fayalite, so the Ti concentration was also introduced into the regression equation as a multiplier for all terms.

After determining the regression equation and each parameter based on these considerations, the regression equation and each parameter were compared with the actual occurrence rate of double lamination in steel sheets produced by heating to 1200°C in a heating furnace and hot rolling. Then, the boundary between the index f of the liquid phase fraction of scale where double lamination does not occur and the index f of the liquid phase fraction of scale where double lamination occurs was normalized to 1.00. An example of the regression equation determined in this way is the above formula (1), and examples of the parameters at that time are A _Ti1 : 8.2, A _C : -0.7, A _Si : 2.0, A _Mn : -0.5, A _Ti2 : 23, and A _Si/Mn : 2.7.

In the above example, the regression equation and parameters for calculating the liquid phase fraction index f of scale are determined by calculating the liquid phase fraction of scale using thermodynamic calculations, but the method for determining the regression equation and parameters for calculating the liquid phase fraction index f of scale is not limited to this. For example, the regression equation and parameters may be determined by actually generating scale in a laboratory experiment and investigating the state of that scale. Furthermore, the regression equation and parameters may be determined from the relationship between the occurrence rate of double-layer crack defects in an actual machine and the component concentrations of molten steel 18.

In the slab assessment method according to this embodiment, an example has been shown in which the component concentrations of molten steel 18 poured into the mold 10 of the continuous casting apparatus 100 are used to assess the risk of a slab about to be produced by the continuous casting apparatus 100 splitting into two pieces, but this is not limited to this. The slab assessment method according to this embodiment can assess the risk of a slab splitting into two pieces not only for slabs about to be produced by the continuous casting apparatus 100, but also for slabs produced in the past. In this case, the component concentrations of the slabs produced in the past can be used instead of the component concentrations of the molten steel 18 poured into the mold 10 to determine an index of the liquid phase ratio of scale that forms on the slab in the high-temperature environment of a heating furnace.

Embodiments of the present invention are not limited to the above-described embodiments and various modifications can be made. In the slab manufacturing method according to this embodiment, an example has been shown in which the continuous casting conditions are changed to apply a soft reduction to a slab 38 determined to have a risk of splitting into two pieces in the second process, but this is not limiting. For a slab 38 determined to have a risk of splitting into two pieces in the second process, the presence or absence of cross-sectional defects in the slab 38 may be determined, and the cross-sectional defects of the slab 38 determined to have cross-sectional defects may be welded. The presence or absence of cross-sectional defects in the slab 38 can be determined using the following two cross-sectional defect determination methods.

<Method 1 for determining cross-sectional defects>
First, a method 1 for determining cross-sectional defects in a slab will be described. As a result of extensive research, the inventors have found that as the temperature difference in the width direction of a slab 24 at the final stage of solidification increases, cross-sectional defects are more likely to occur in a slab 38 produced by cutting the slab 24. The temperature difference in the width direction of the slab 24 is measured in the range from the position where the slab support rolls 32 are no longer provided in the casting direction to the position where the slab cutter 36 is provided.

Figure 3 is a graph showing the relationship between the temperature difference across the width and cross-sectional defects of the slab. In Figure 3, the horizontal axis represents the width dimension (mm) of the slab, and the vertical axis represents the temperature difference across the width (°C). The temperature difference across the width is the temperature difference between the maximum temperature within a range of ±100 mm from the center of the slab 24 across the width and the maximum temperature across the entire width of the slab (excluding the range of ±100 mm from the center of the width). In the graph, "○" indicates an example of a slab 38 that was free of cross-sectional defects, and "×" indicates an example of a slab 38 that was found to have cross-sectional defects.

As shown in Figure 3, when the temperature difference in the width direction of the slab 24 is 80°C or more, cross-sectional defects are more likely to occur in the slab 38, and when the temperature difference in the width direction of the slab 24 is 100°C or more, cross-sectional defects occur in the slab 38. From these results, it can be seen that the presence or absence of cross-sectional defects in the slab 38 can be determined by using the temperature difference in the width direction of the slab 24 from the completion of solidification of the slab 24 to the position where the slab cutting machine 36 is installed.

Specifically, if the second process determines that the slab is at risk of splitting into two pieces, the temperature difference across the width of the slab 24 is measured using a thermal camera 44, and if the temperature difference exceeds 80°C, it is determined that a cross-sectional defect has occurred in the slab 38 to be produced. On the other hand, if the temperature difference is 80°C or less, it is determined that no cross-sectional defect has occurred in the slab 38 to be produced.

A temperature difference of 80°C is an example of a predetermined temperature difference threshold, and is determined in advance by investigating the relationship between the temperature difference in the width direction and the presence or absence of cross-sectional defects, as shown in the graph in Figure 3. In this way, in the slab manufacturing method of this embodiment, the presence or absence of cross-sectional defects may be determined for slabs determined to have a risk of splitting into two pieces in the second process using cross-sectional defect determination method 1.

<Method 2 for determining cross-sectional defects>
Next, a method 2 for determining a cross-sectional defect in a slab will be described. In the method 2 for determining a cross-sectional defect, the presence or absence of a cross-sectional defect in the slab 38 is determined using image data generated by an imaging device capturing an image of the cut surface cut by the slab cutting machine 36. When a cross-sectional defect occurs in the slab 38, a crack occurs in the cut surface of the slab 38. Therefore, the presence or absence of a cross-sectional defect in the slab 38 can be determined by detecting the crack in the cut surface using image data generated by capturing an image of the cut surface of the slab 38.

Cracks can be detected by an operator visually inspecting the image data, or by processing the image data. Because cracks have a lower brightness than other parts, the image data can be binarized using a brightness threshold that can distinguish between cracks and other parts to determine the width and height dimensions of the crack, and the presence or absence of a crack can be detected by comparing these dimensions with the crack's past actual dimensions.

It is preferable that the imaging device 48 has an IR cut filter. Because the cut surface of the slab 38 is red-hot, if the cut surface is directly imaged, the difference in pixel brightness between the crack area and the pixel brightness in other areas will be small, making it difficult to detect the crack. In contrast, by using an imaging device equipped with an IR cut filter that cuts light with wavelengths of 700 nm or more, the effect of red-hot heat can be reduced, enabling reliable detection of cracks on the cut surface.

In addition, it is preferable to illuminate the cut surface of the slab 38 using the lighting device 46. Furthermore, it is preferable that the lighting device 46 illuminate the cut surface of the slab 38 at an angle from either above or below the thickness direction of the slab 38. In this way, by illuminating the cut surface from the lighting device 46 at an angle, a shadow is cast on the crack, allowing for more stable crack detection.

If a cross-sectional defect is present in a slab determined to have a risk of splitting in two in the second process, there is a high possibility that splitting will occur in the hot-rolled steel sheet produced by hot rolling. Therefore, if a cross-sectional defect is determined to exist using cross-sectional defect determination method 1 or 2, the crack that has occurred on the cut surface of the slab 38 is welded to close the gap. This prevents scale with a high liquid phase ratio from entering the gap in the cross-sectional defect, making it possible to prevent splitting in two in the hot-rolled steel sheet produced by hot rolling, even if the slab is determined to have a risk of splitting in two and a cross-sectional defect.

On the other hand, if it is determined that there are no cross-sectional defects using cross-sectional defect determination method 1 or 2, the hot-rolled steel plate manufactured using that slab 38 will not split into two pieces. Therefore, a slab 38 determined to have no cross-sectional defects using cross-sectional defect determination method 1 or 2 may be used as is to manufacture hot-rolled steel plate. By making a determination using cross-sectional defect determination method 1 or 2 in this way, it is possible to reduce the number of slabs removed from the production line to weld the cut surfaces of slab 38, thereby suppressing increases in slab manufacturing costs.

The following describes examples in which the effects of the present invention were confirmed. Slabs were produced using the continuous casting device 100 for steel types A to C. The component compositions of steel types A to C, the index f of the scale liquid phase ratio, whether or not soft reduction was performed, and the incidence of double cracking are shown in Table 1 below. The index f of the scale liquid phase ratio is a value calculated using the following formula (2).

f=8.2×[Ti]×{-0.7×[C]+2.0×[Si]-0.5×[Mn]+23×[Ti]+2.7×([Si]/[Mn]) ^0.5 }...(2)

In Table 1, multiple tests are conducted on the same steel grade in order to evaluate variations in chemical composition. In Table 1, the double crack occurrence rate indicates the percentage of hot-rolled steel sheets manufactured using each slab that experienced double cracks.

Tests Nos. 1-3 and 5-7 demonstrate the validity of assessment using the scale liquid fraction index f due to component variation. As shown in Table 1, when a slab with a scale liquid fraction index f of 1.00 or higher was used, the slab was heated to 1200°C in a heating furnace and hot-rolled to produce a hot-rolled steel sheet, and a double-lamination occurred. On the other hand, when a slab with a scale liquid fraction index f of less than 1.00 was used, the slab was heated to 1200°C in a heating furnace and hot-rolled to produce a hot-rolled steel sheet, and a double-lamination did not occur. Based on these results, the component concentrations of the molten steel before being cast in a continuous casting machine were used to calculate the scale liquid fraction index f of the slab produced from that molten steel. It was then confirmed that the risk of double-lamination in a hot-rolled steel sheet produced by hot-rolling the slab can be assessed by determining whether the calculated index f is equal to or greater than a predetermined threshold.

Test Nos. 4, 8, and 9 show the results of soft reduction under modified casting conditions for slabs that were assessed as having a risk of splitting into two pieces, with a scale liquid phase ratio index f of 1.00 or higher. Although test Nos. 4, 8, and 9 were assessed as having a risk of splitting into two pieces, soft reduction was applied to eliminate cross-sectional defects in the slabs. The soft reduction rates applied were No. 4: 0.24 mm/min, No. 8: 0.66 mm/min, and No. 9: 0.96 mm/min. Thus, for slabs Nos. 4, 8, and 9 that had been soft reduced, the slabs were heated to 1200°C in a heating furnace and hot-rolled to produce hot-rolled steel sheets, and no splitting occurred in the hot-rolled steel sheets. These results confirmed that by applying appropriate light reduction to slabs determined to be at risk of splitting into two pieces at the end of solidification, it is possible to prevent splitting into two pieces in the hot-rolled steel plate produced by hot rolling the slab.

REFERENCE SIGNS LIST 10 Mold 12 Tundish 14 Sliding nozzle 16 Submerged entry nozzle 18 Molten steel 20 Solidified shell 22 Unsolidified layer 24 Strand 26 Support roll 28 Guide roll 30 Pinch roll 32 Strand support roll 34 Transport roll 36 Strand cutter 38 Slab 40 Solidification completion position 42 Soft reduction zone 44 Thermocamera 46 Lighting device 48 Imaging device 100 Continuous casting device

Claims (18)

A method for determining a slab containing Ti, C, Si, and Mn as components and produced using a continuous casting machine, comprising:
a first step of determining an index of a liquid phase ratio of scale formed on the slab using concentrations of components of molten steel poured into a mold of a continuous casting machine;
A second step of determining that the slab is at risk of splitting into two pieces when the index of the liquid phase ratio of the scale is equal to or greater than a predetermined threshold value;
and
The method for evaluating a slab, wherein the index of the liquid phase ratio of the scale is an index determined by the Ti concentration, the C concentration, the Si concentration, and the Mn concentration of the molten steel. The slab evaluation method according to claim 1 , wherein the index of the liquid phase ratio of the scale is an index determined by the following formula (1):
f=A _Ti1 × [Ti] × {A _C × [C] + A _Si × [Si] + A _Mn × [Mn] + A _Ti2 × [Ti] + A _Si/Mn × ([Si]/[Mn]) ^0.5 }... (1)
In the above formula (1), f is an index (-) of the liquid phase rate of the scale, [Ti] is the Ti concentration (mass%) of the molten steel, [C] is the C concentration (mass%) of the molten steel, [Si] is the Si concentration (mass%) of the molten steel, [Mn] is the Mn concentration (mass%) of the molten steel, and A _Ti1 , A _C , A _Si , A _Mn , A _Ti2 and A _Si/Mn are parameters. A slab assessment method as described in claim 1 or claim 2, wherein in the second step, the slab is assessed as a slab with no risk of splitting into two pieces if the index of the liquid phase ratio of the scale is less than a predetermined threshold value. A method for producing a slab using a continuous casting device, comprising:
A method for manufacturing a slab, wherein when a slab is determined to have a risk of splitting into two pieces using the slab determination method described in claim 1 or 2, the roll opening of the reduction rolls of the continuous casting device is changed so that the solid phase ratio at the center of the width and center of the thickness of the slab in the casting direction is in the range of greater than 0.0 and not more than 1.0, thereby lightly reducing the slab. A method for producing a slab using a continuous casting device, comprising:
A slab manufacturing method, in which, in the slab judgment method described in claim 1 or 2, if a slab is judged to have a risk of splitting into two pieces, the temperature difference in the width direction of the slab is measured in the range from the position where the slab support rolls are no longer installed to the position where the slab is cut to judge whether or not there is a cross-sectional defect. A method for producing a slab using a continuous casting device, comprising:
A slab manufacturing method in which, in the slab judgment method described in claim 1 or 2, if a slab is judged to be at risk of splitting into two pieces, the cut surface of the slab is imaged to generate image data, and the presence or absence of cross-sectional defects is judged using the image data. The method for manufacturing a slab described in claim 6, wherein the cut surface is imaged using an imaging device having an IR cut filter. The slab manufacturing method described in claim 5, wherein the gap in a cross-sectional defect of a slab determined to have such a cross-sectional defect is welded. The slab manufacturing method described in claim 6, wherein, for slabs determined to have cross-sectional defects, the gaps in the cross-sectional defects are welded. The slab manufacturing method described in claim 7, wherein, for slabs determined to have cross-sectional defects, the gaps in the cross-sectional defects are welded. A method for manufacturing a steel plate by heating a slab and hot rolling it,
A method for manufacturing a steel plate, comprising heating and hot rolling a slab that has been determined to have no risk of splitting into two pieces in the slab determination method according to claim 3. A method for manufacturing a steel plate by heating a slab and hot rolling it,
A method for producing a steel plate, comprising heating and hot rolling a slab produced by the method for producing a slab according to claim 4. A method for manufacturing a steel plate by heating a slab and hot rolling it,
A method for producing a steel plate, comprising heating and hot rolling a slab determined to be free of cross-sectional defects by the method for producing a slab according to claim 5. A method for manufacturing a steel plate by heating a slab and hot rolling it,
A method for producing a steel plate, comprising heating and hot rolling a slab determined to be free of cross-sectional defects by the method for producing a slab according to claim 6. A method for manufacturing a steel plate by heating a slab and hot rolling it,
A method for producing a steel plate, comprising heating and hot rolling a slab determined to be free of cross-sectional defects by the method for producing a slab according to claim 7. A method for manufacturing a steel plate by heating a slab and hot rolling it,
A method for producing a steel plate, comprising heating and hot rolling a slab produced by the method for producing a slab according to claim 8. A method for manufacturing a steel plate by heating a slab and hot rolling it,
A method for producing a steel plate, comprising heating and hot rolling a slab produced by the method for producing a slab according to claim 9. A method for manufacturing a steel plate by heating a slab and hot rolling it,
A method for producing a steel plate, comprising heating and hot rolling a slab produced by the method for producing a slab according to claim 10.